In radio communication systems for use in microwave band, sub-millimeter wave band, millimeter wave band or the like, there has been widely used a traveling-wave array antenna in which antenna elements are arrayed along a feeder line. In this traveling-wave array antenna, the energy of a transmitting signal travels along the feeder line toward its terminating portion, where a part of the energy is successively radiated so as to be transmitted in a predetermined direction. This traveling-wave array antenna has such a feature that the circuit design of the feeder line is relatively easy.
FIG. 28 is a circuit diagram showing a constitution of a traveling-wave array antenna apparatus 504 according to a prior art.
Referring to FIG. 28, the traveling-wave array antenna apparatus 504 has a plurality of antenna elements 503 arrayed on a feeder line 502 along its longitudinal direction. In this arrangement, an electromagnetic wave inputted via a feeding portion 501 travels along the feeder line 502 toward its terminating portion in a direction of arrow 502a, feeding power successively to each of the plurality of antenna elements 503, so that the electromagnetic wave is radiated from each of the antenna elements 503 in a predetermined radiating direction.
The excitation amplitude of each antenna element 503 can be controlled by changing the size and configuration of each antenna element 503 of this traveling-wave array antenna 504, while the excitation phase of each antenna element 503 can be controlled by changing the interval between the adjacent elements of the antenna elements 503. By controlling excitation coefficients each including an excitation amplitude and an excitation phase, the desired radiating directivity characteristic can be obtained.
For example, in base station antennas for use in a subscriber radio system such as a so-called FWA (Fixed Wireless Access) system, an array antenna is often used to form a vertical-plane radiating directivity characteristic, where excitation coefficients of the array antenna are controlled to form a vertical-plane radiating directivity characteristic of a cosecant-squared curve, thus making it possible for respective subscriber radio stations to transmit and receive substantially the same power.
FIG. 29 is a perspective view showing a constitution of a waveguide slot array antenna apparatus 508, which is an example of the traveling-wave array antenna apparatus of FIG. 28.
Referring to FIG. 29, the waveguide slot array antenna apparatus 508 is provided with slot antennas 507 implemented by forming a plurality of rectangular slots, respectively, in a top surface of a rectangular waveguide 506 serving as a feeder line. A rectangular-shaped input opening 505 is formed at a bottom surface so as to close to one terminating portion of the rectangular waveguide 506. A rectangular waveguide 509 of a feeder line is connected to the input opening 505.
In the waveguide slot array antenna apparatus 508 constructed as shown above, a transmitting electromagnetic wave is transmitted from a radio transmitter via the rectangular waveguide 509, and thereafter, is inputted to the rectangular waveguide 506 via the input opening 505. Then, the electromagnetic wave propagates along the longitudinal direction of the rectangular waveguide 506 toward the other terminating portion, and the propagating electromagnetic wave is radiated via the rectangular slots of the slot antennas 507.
In this waveguide slot array antenna apparatus 508, since the use of a rectangular waveguide eliminates the radiation from the feeder line, the loss of the feeder line can be reduced. Further, the excitation amplitude can be controlled by changing the length or width of the rectangular slot of each slot antenna 507, and the excitation phase can be controlled by changing the interval between the adjacent antennas located between the respective rectangular slots, and thus a desired radiating directivity characteristic can be obtained by controlling excitation coefficients each including the excitation amplitude and the excitation phase. Accordingly, it is simple to form an array antenna having the desired radiating directivity characteristic. Therefore, the waveguide slot array antenna apparatus 508 is an array antenna apparatus effective for microwave band, in particular, millimeter wave band.
However, with the construction of the prior art shown in FIGS. 28 and 29, when the frequency of the transmitting electromagnetic wave is changed, the phase delay of the propagating traveling wave between the antenna elements 503 is also changed due to change in guide wavelength within the feeder line 502. Also, in the case of the waveguide slot array antenna apparatus 508, since the traveling wave propagating along the rectangular waveguide 506 passes just under the slot antennas 507, the passed transmitted wave also has a phase delay and is changed in transmitted phase depending on the frequency of the electromagnetic wave. For these reasons, the phase given to the electromagnetic wave radiated from each antenna element 503 or 507 is changed, so that the excitation phase of each antenna element 503 or 507 is changed.
In these array antenna apparatuses 504 and 508, because of a power feeding technique such as the traveling-wave feeding technique as described above, the farther the antenna element is from the power feeding section so as to be close to the input opening 505, the more those phase changes would be accumulated, causing a larger phase change to be given to the radiated electromagnetic wave. Accordingly, occurrence of change in phase difference between the antenna elements 503 or 507 would cause the direction of the main beam of the radiating directivity characteristic of the antenna apparatuses 504 and 508 to change.
For example, in the case where these traveling-wave array antenna apparatuses 504 and 508 are used at a base station of the FWA system, occurrence of change in the main beam direction would cause decrease in the intensity of the received signal at subscriber radio stations present at marginal end portions of the service area as well as falls in the substantial transmitting signal power at those subscriber radio stations.
An object of the present invention is to solve the above-mentioned problems, and to provide a traveling-wave array antenna apparatus capable of suppressing the change in the main beam direction of the radiating directivity characteristic for change in frequency in the transmitting electromagnetic wave.